A like brake system is known e.g. from DE 36 27 000 A1. Furthermore, methods are already known and made use of in automotive vehicles, wherein in each case the braking pressure of the imminently locking wheels is regulated by virtue of the inlet valves and outlet valves as well as the electronic circuitries. The braking pressure on the wheels devoid of a tendency to lock remains unaffected.
In the majority of cases, admittedly, the known control methods for operating a brake system like described above are very effective and guarantee safe braking and mastering of the automotive vehicle. However, an undesirable situation will be encountered under extreme conditions.
When gentle braking is initiated for instance on a snowy road surface, that means the brake pedal is depressed only a small amount, and if any one wheel, e.g. VR, shows a tendency to lock, the pumps will be driven to deliver pressure fluid out of the supply reservoir into the brake circuits. The braking pressure is regulated on the wheel VR showing the tendency to lock. Since the pumps are designed such that they return a sufficient amount of pressure fluid into the brake circuits even if control is effected on all wheels, in the event of only one wheel being controlled they will deliver so much pressure fluid into the brake circuits that the pressure rises there. Due to the pressure being increased in the master cylinder as well, augmented force will react on the foot of the driver placed on the brake pedal. However, since the average driver applies the brake pedal not in a force-proportional manner but in a travel-proportional way, he/she will yield the higher pressure after a few seconds only and will not release the pedal until then.
In the meantime, that means after the pressure increase and before the pedal release, the following happens:
Caused by the increased braking pressure, even the wheels which had not been jeopardized by wheel lock so far may reach the limit of wheel lock and will then be governed corresponding to the prevailing high braking pressure level. The driver had not intended to brake so strongly and may get startled. In case another vehicle rides behind such an unexpectedly vigorously braked automotive vehicle, an accident will possibly occur.